1. Field
The present disclosure relates to non-volatile storage devices.
2. Description of the Related Art
Materials having a detectable level of change in state, such as a resistance, are used to form various types of non-volatile semiconductor based memory cells. It has been proposed that such memory cells might be used for binary data storage in memory arrays by assigning a lower resistance state of a memory cells to a first logical state such as logical ‘0,’ and assigning a higher resistance state of the memory cell to a second logical state such as logical ‘1.’ Other logical data assignments to resistance states may also be used. Some materials can be reset back to a higher resistance state after being set from an initial state to a lower resistance state. These types of materials can be used to form re-writable memory cells. Multiple levels of detectable resistance in materials might be used to form multi-state memory cells which may or may not be re-writable.
One type of memory cell that exhibits switching behavior between at least two resistance states is referred to as ReRAM for “resistive switching RAM”. ReRAM may also be referred to as R-RAM or RRAM. A ReRAM memory cell may include a first electrode, a re-writable switching material (also referred to as a state change element), and a second electrode. The switching material may be metal oxide (MeOx). Switching the memory cell between resistance states may be achieved by applying a voltage across the memory cell. An alternative way of explaining the switching between resistance states is to provide a current to the memory cell.
One theory that is used to explain the switching mechanism is that one or more conductive filaments are formed by the application of a voltage (or other signal) to the memory cell. For some memory cells, the conductive path may arise due to oxygen vacancies that are caused by application of the voltage. This path (or paths) may link the first and second electrodes, wherein the conductive filament(s) lowers the resistance of the memory cell. Application of another voltage may rupture the conductive filament(s), thereby increasing the resistance of the memory cell. Application of another still another voltage may repair the rupture in the conductive filament(s), thereby decreasing the resistance of the memory cell once again.
The reversible resistivity-switching element may be in the high resistance state when it is first fabricated. The term “forming” is sometimes used to describe putting the reversible resistivity-switching element into a lower resistance state for the first time. Thus, the initial formation of the conductive filaments is sometimes referred to as “forming.” The rupture of the filaments is sometimes referred to as RESETTING. The repair of the rupture of the filaments is sometimes referred to as SETTING.
For various reasons, it may be desirable to reduce the voltage that is needed to switch the memory cell from one state to the other. It may also be desirable to reduce the power needed to switch the memory cell state. It may also be desirable to reduce the current needed to switch the memory cell state. For example, reducing the current that is needed to reset the memory cell may be beneficial in that it may reduce disturb issues and reduce stress on the memory cell. Reducing stress may increase the number of set/reset cycles that are possible. Reducing the required reset current can also reduce power requirements.